Method of making mechanical pulp

ABSTRACT

A method for producing mechanical pulps which are useful for producing coated light weight paper, magazine paper and the like, is disclosed, which includes preheating lignocellulose-containing material in the presence of impregnation chemicals, refining the preheated lignocellulose-containing material utiilizing a pair of counter-rotating refining discs under pressure, bleaching the thus produced partially refined lignocellulose-containing material, and refining the bleached material in a second refining step utilizing a pair of refining discs, one of which is stationary and the other which is rotating under pressure.

FIELD OF THE INVENTION

The present invention relates to methods of manufacturing mechanicalpulps. More particularly, the present invention relates to methods ofmanufacturing mechanical pulps from lignocellulose-containing materials,and particularly in which the pulps are intended for the production ofcoated paper having a low grammage, such as so-called LWC paper (lightweight coated), magazine paper or similar such paper.

BACKGROUND OF THE INVENTION

In connection with the production of such coated papers or LWC papers,the properties of the pulps so utilized must meet certain very strictrequirements. The reason for this is primarily because the paper musthave high density, low roughness, low porosity, as well as highstrength. Furthermore, a uniform surface structure of these papers is ofparticular importance.

Paper of this type normally includes both chemical and mechanical pulps.The traditional mechanical pulp component used in this paper has been agroundwood pulp. However, as an alternative, thermomechanical pulp (TMP)has been used in recent years, but with limited success. Experience hasthus proven that the employment of such TMP as a mechanical pulpcomponent, even at low freeness values, results in a certain unevennessin the surface structure of the paper. This, in turn, gives rise to poorcoating and, thus, to unacceptable printability. It has only beenpossible until now to avoid these problems by the paper manufacturertaking special steps to either modify or eliminate entirely the negativeeffects of that fraction in the thermomechanical pulp which constituteslong fibers. This long fiber fraction thus includes long, rigid andunworked fibers, which have a negative effect on the surface structureof the papers produced therefrom.

Most recently, chemically modified TMP (CTMP) has arisen as anattractive alternative to TMP as a component of the mechanical pulp insuch LWC-papers and papers of similar quality. It has thus been foundthat CTMP improves the strength and binding properties thereof, andpermits a reduction in the aforenoted problems with long fiberfractions. This is due to the fact that the mild chemical pretreatmentused therein substantially improves both the defibering capability ofthe wood material and the tendency of the fibers in the long fiberfraction of the pulp to swell. These fractions, in turn, provide for alower shives content and improved flexibility and ductility for the longfibers in CTMP, as compared to TMP.

Both the TMP and CTMP intended for use in LWC-papers and the like aregenerally manufactured by refining in one or more steps, and thenscreening and bleaching the refined material. The energy consumption insuch a process is relatively high, and considerable equipment fordewatering and washing is required. Therefore, the search has continuedfor improved methods of producing such papers.

SUMMARY OF THE INVENTION

In accordance with the present invention, these and other objects,including simplification of the equipment and reduction in the energyconsumption while maintaining, or even improving, pulp quality and theproduction of such coated papers and the like have now beenaccomplished. More particularly, in accordance with this invention, thishas now been accomplished by applicants' discovery of a method forproducing a mechanical pulp product useful for the production of coatedpaper and the like from lignocellulose-containing material whichincludes preheating the lignocellulose-containing material in thepresence of impregnation chemicals so as to produce a preheatedlignocellulose-containing material, refining the preheatedlignocellulose-containing material in a first refining step employing apair of counter-rotating refining discs under pressure so as to producea partially refined lignocellulose-containing material, bleaching thepartially refined lignocellulose-containing material in the presence ofbleaching chemicals of high concentrations so as to produce a bleachedlignocellulose-containing material, and refining the bleachedlignocellulose-containing material in a second refining step utilizing apair of refining discs in which one of the pair of refining discs isstationary and the other of the pair of refining discs is rotating underpressure so as to produce a refined lignocellulose-containing material.

In accordance with a preferred embodiment of the method of the presentinvention, the method includes screening the refinedlignocellulose-containing material. In another embodiment, refining inthe first refining step is carried out so as to produce a partiallyrefined lignocellulose-containing material having a freeness valuemeasured by CSF of between about 150 and 300. In a preferred embodiment,the refining in the second refining step is carried out so as to producea refined lignocellulose-containing material having a freeness valuemeasured by CSF of between about 50 and 150. In a particularly preferredembodiment, the bleaching step is carried out employing a bleachingchemical such as a peroxide or a dithionite, and the bleaching step iscarried out in order to produce a bleached lignocellulose-containingmaterial having an ISO-brightness above about 70. In a particularlypreferred embodiment, preheating is carried out in the presence of animpregnation chemical such as sodium sulfite or sodium bisulfite.Preferably, the preheating step is carried out at a PH in the range ofabout 6 and 12.

In accordance with another embodiment of the method of the presentinvention, preheating of the lignocellulose-containing material iscarried out at a preheating temperature of between about 105° and 145°C. In accordance with another embodiment, the bleaching step is carriedout at a concentration of between about 30 and 45%, and in anotherembodiment, refining in the second refining step is carried out at aconcentration of between about 25 and 45%.

In accordance with the overall method of the present invention, therefining process is thus carried out in two separate steps with anintermediate bleaching step therebetween. Therefore, subsequent toimpregnation and preheating of the raw material in the form of woodchips, initial refining is carried out in a double-disc refiner, thatis, a refiner in which two counter-rotating refining discs are utilized.Bleaching of the pulp at high concentration is then carried out and,subsequently, a second refining step is conducted at high concentrationin a disc-refiner of the single-disc type, i.e., one with one stationaryand one rotating disc. In a preferred embodiment, the pulp is thensubjected to screening. In accordance with this invention, thedevelopment of the light-scattering coefficient is maximized in thefirst refining step. It is thus generally known that double-discrefiners yield a higher light scattering coefficient than do single-discrefiners. Furthermore, due to the subsequent use of the bleaching step,the pulp is then easily dewatered so that the high pulp concentrationrequired for bleaching can be obtained with a single dewateringapparatus. Furthermore, energy consumption can be reduced on an overallbasis, since the second refining step can be carried out on pulp whichhas already been bleached. The Shives content of the pulp can also beminimized by utilizing a single disc refiner in that final refiningstep.

DETAILED DESCRIPTION

The present invention can be more fully appreciated with reference tothe following detailed description. In particular, in a preferredembodiment, raw materials in the form of wood chips are subjected topretreatment by washing, chemical impregnation and preheating, all in aconventional manner. It is preferred that sodium sulfite (Na₂ SO₃) ormixtures of sodium sulfite and sodium busilfite (NaHSO₃) be utilized asthe impregnation chemicals, preferrably within the pH range of about 6to 12. The temperature of the lignocellulose materials are increased upto temperatures of from 105° to 145° C. in the preheating step.

Subsequent to pretreatment in this manner, the material is thensubjected to refining under pressure in a double-disc refiner. Refiningin this manner in this first step thus yields a pulp with a freenessvalue, according to CSF, of from about 150 to 300. This pulp is thenrelatively easy to dewater. The pulp can thus be pumped to the bleachingstep, where it is dewatered to the desired concentration, preferablyfrom about 30 to 45%. The dewatering equipment can therefore be simple,and there is thus a rather low investment cost associated therewith.Thereafter, the chemicals required for the bleaching are added.Bleaching is preferably carried out with peroxide or dithionite, to anISO-brightness of greater than about 70.

After the bleaching step, the pulp is washed, and then subjected to thesecond refining step, which is carried out at high concentration,generally at between about 25 and 45%, and is driven to a freenessvalue, according to CSF, of from about 50 to 150.

Since the second refining step is carried out subsequent to bleaching,the energy consumption for such refining is reduced. The bleaching stephas thus rendered the pulp easier to be processed to the desiredquality. The second refining step is preferably carried out under anoverpressure in a single-disc refiner which is preferably equipped witha device for obtaining accurate gap adjustment and gap control, and inthis manner it is possible to minimize the shives content of the pulp,and at the same time control the shortening of the fiber length to thedesired mean fiber length of the pulp, at the desired energyconsumption, at least within certain limits. Subsequent to the secondrefining step, the pump is screened and cleaned, by conventional means,before the pulp is then ready to be transferred to a paper mill for themanufacture of LWC-paper, or paper of similar quality.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

We claim:
 1. A method for producing a mechanical pulp product fromlignocellulose-containing material comprising pretreating saidlignocellulose-containing material in the presence of impregnationchemicals so as to produce a pretreated lignocellulose-containingmaterial, refining saidpretreated lignocellulose-containing material ina first refining step employing a pair of counter-rotating refiningdiscs under pressure so as to produce a partially refinedlignocellulose-containing material having a freeness value measured byCSF of between about 150 and 300 so as to maximize the light-scatteringcoefficient of said lignocellulosecontaining material, bleaching saidpartially refined lignocellulose-containing material in the presence ofbleaching chemicals at a pulp concentration of between about 30 and 45percent so as to produce a bleached lignocellulose-containing materialhaving an ISO-brightness above about 70, and refining said bleachedlignocellulose-containing material in a second refining step utilizing apair of refining discs in which one of said pair of refining discs isstationary and the other of said pair of refining discs is rotatingunder pressure so as to produce a refined lignocellulose-containingmaterial having a freeness value measured by CSF of between about 50 and150 so as to minimize the shives content of said refinedlignocellulose-containing material.
 2. The method of claim 1 includingscreening said refined lignocellulose-containing material.
 3. The methodof claim 1 wherein said bleaching is carried out employing a bleachingchemical selected from the group consisting of peroxide and dithionate.4. The method of claim 1 wherein said pretreating of saidlignocellulose-containing material is carried out in the presence of animpregnation chemical selected from the group consisting of sodiumsulphite and sodium bisulphite.
 5. The method of claim 4 wherein saidpretreating of said lignocellulose-containing material is carried out ata pH in the range of between about 6 and
 12. 6. The method of claim 1wherein said pretreating of said lignocellulose-containing materialcomprises preheating of said lignocellulose-containing material at apreheating temperature of between about 105° and 145° C.
 7. The methodof claim 1 wherein said refining of said bleachedlignocellulose-containing material in said second refining step iscarried out at a pulp concentration of between about 25 and 45 percent.